Baboons will be vaccinated with a highly irradiated, cryopreserved, Schistosoma mansoni schistosomule vaccine. Development of immunity to a percutaneous challenge infection with normal S. mansoni cercariae will be assessed by determining adult worm burdens in vaccinated and unvaccinated groups of baboons. The groups will also be compared with respect to other parasitologic criteria, including fecal egg excretion, tissue egg loads, and oviposition sites; and with respect to immunologic reactivity with mitogens and schistosome antigens. The latter will include measurements of circulating antibodies, immune complexes, schistosomal antigens, complement levels, lymphocyte transformation, and leukocyte-mediated damage to parasites in vitro. This experiment should yield valuable data with respect to efficacy of a highly promising schistosomiasis vaccine in a stringent animal model, as well as yield immunologic data that may further understanding of immunity to schistosomiasis in primates (including man).